Workers in the art of making and/or using web substrates having a thin surface coating are aware that it gets increasingly difficult to maintain a high degree of coating uniformity as thickness decreases. For instance, with magnetic recording tape or with floppy disks cut therefrom, it is becoming increasingly important to provide "ultra-thin" magnetic recording films (on the order of 1 micron thick or less; yet, it is extremely difficult, and relatively costly, to make such "ultra-thin" coatings truly uniform (in the sense understood in precision magnetic recording). It is an object of this invention to answer this need.
Prior Art: "Paired Metal Roll" array; FIG. 1:
FIG. 1 indicates, very schematically, a technique for applying a thin film coating of magnetic oxide to a magnetic recording tape substrate T (e.g., a polyethylene teraphthalate tape) using a "paired metal roll" delivery arrangement known in the art. That is, a known liquid base magnetic oxide coating mixture m (e.g., slurry of iron oxide particles in organic carrier) is taken up from a suitable storage vessel C and applied to moving web substrate T in a prescribed controlled manner to yield a thin coating, with thickness uniformity being rather precisely controlled (hopefully)--e.g., an oxide film of several hundred microns or less to be applied to tape for use in precision magnetic recording. Such a coating operation may be understood as here carried-out by a pair of metal coating rolls, D.sub.1, D.sub.2, with D.sub.2 functioning as the "applicator roll" (delivering mix m from vessel C to tape T) and D.sub.1 serving as "trimmer", while an associated roll D.sub.3 drives the tape.
More particularly, a first, rigid metal-surfaced (e.g., steel) applicator roll D.sub.2 is continually rotated and, as indicated in FIG. 1, is positioned to dip into mix m in container C, then pick-up a surface deposit of the mix and carry it past trimmer roll D.sub.2, to coat substrate T. D.sub.2 will therefore be understood as rotationally swept, at a prescribed surface velocity, past a prescribed "transfer site" P-1--here identified as the intersection of the surface of drum D.sub.2 with reference "trim-plane" T--T, tangent to D.sub.2 --whereat the deposit is "trimmed" by D.sub.1 to control deposit-thickness.
Similarly, companion rigid, metal-surfaced, trim roll D.sub.1 is counter-rotated past D.sub.2, near the same point of tangency, P-1, but separated therefrom (i.e., plane T--T) by a prescribed "trim-space" S as mentioned below. Roll D.sub.1 is counter-rotated at roughly the same surface speed as D.sub.2, to be swept, opposingly, past the surface of roll D.sub.2, close enough to effect a thickness-control of the deposit on roll D.sub.2, spacing S being adjusted to control the deposit thickness and this controls the thickness (and thickness-uniformity) of the film transferred onto passing substrate web T, as understood in the art. Roll D.sub.1 is provided with a doctor blade DB, or similar arrangement, for conventionally wiping its surface clean of any deposit of mix m and returning the mix to vessel C below.
Web T will be understood as driven past roll D.sub.2 (i.e., past the "coating-site" P-2 whereat mix m is transferred to web T) by suitable resilient-surface drive roll D.sub.3 (preferably covered with rubber or a like elastomer to accommodate a resilient, transfer-engagement against the web and roll D.sub.2), pressing web T firmly against the rigid surface of roll D.sub.2 as known in the art. Web T may be, essentially, endless and of any convenient width (e.g., 6"-48" widths are well known in the magnetic tape art) and driven at a relatively conventional rpm (100-1000 ft./min. surface-speed preferred here for rolls D.sub.1, D.sub.2 and D.sub.3).
The constituents of mix m will be known in the art--e.g., magnetic recording oxide particles in an organic vehicle like acetone, methyl ethyl ketone (MEK), a tetrahydro-furan (THF) methyl iso-butyl ketone (MIBK) or cyclohexanone. Variations in solvent concentration can be very troublesome; e.g., leading to difficulty in maintaining uniform coating thickness (such as when solvent concentration changes because of evaporation under extended use in a warm environment).
Workers are aware that, with such a coating arrangement, the thickness of the coating carried (past roll D.sub.1) on roll D.sub.2 will, in turn, determine the thickness of the coating delivered to substrate T--and that this thickness (on the surface of D.sub.2) is, in turn, determined by such factors as: mix characteristics (e.g., viscosity, percent solvent, etc.), relative velocities of drum surfaces D.sub.1 /D.sub.2, and, particularly, by the magnitude of inter-drum trim-spacing S. Spacing S may be controlled, conventionally, by shifting the position of roll D.sub.1 toward or away from roll D.sub.2 as indicated in FIG. 1 (see the arrows extending from associated drum-shaft SH-1. This spacing would typically vary between 100 u-in. and 1000 u-in; i.e., 100-1000 microinches).
One serious problem in maintaining the uniformity of such ultra-thin coatings stems from unwanted, or uncontrolled, variations in trim-spacing S, ("ultra-thin" coatings being understood as having a highly-uniform thickness, on the order of from less than about 1 micron to about 100 microns or more). These variations can, at times, extend entirely along the length of the interface between the rolls D.sub.1, D.sub.2 (e.g., as caused by an unintended taper along one or both rolls D.sub.1, D.sub.2, or by imperfections or wear in the roll mounts--e.g., worn bearings or by lack of perfect concentricity between roll shaft and outer surface).
Workers will also recognize that it is virtually impossible to eliminate all such spacing anomalies, practically speaking. The present invention is directed toward an improved roll arrangement for applying such coating materials--improvements which eliminate, or at least ameliorate, such coating thickness anomalies.
Prior Art: "Gravure-rolls"; FIG. 2:
FIG. 2 illustrates, very schematically, a second known arrangement for applying such coating mixtures to tape or like web substrates: i.e., a "gravure roll" array (or "Offset Gravure"), typically involving an engraved applicator roll and an "anilox" (or driver) roll as known in the art.
Such a "gravure roll" is illustrated at D.sub.g and will be understood as having a "ridged surface", with steel bars, or a like pattern of rigid dikes (PP) protruding from the roll surface as an array of surface ridges and intervening depressions--the depressions being adapted to pick up, and retain, coating material m from a suitable associated container C.sub.a. This arrangement will be understood as intended to apply coating material relatively evenly upon the surface of a passing web T.sub.a, driven past by an associated drive roll D.sub.b. Preferably, drive roll D.sub.b is resilient-surfaced (elastomer-coated, typically) so as to press the web surface (at least slightly) into the said gravure-depressions for good contact with the mix m carried therein. A doctor blade device DB, or like leveler, is typically provided to assure that the coating material so-carried in the depressions does not rise above the (uniform) level of the ridges, as well known in the art.
Workers know that, while such a delivery system alleviates some of the problems of other systems (such as that of FIG. 1), it unfortunately generates some problems of its own; such as "streaking" and "solvent-incompatibility". By "streaking" it is meant that the coating is applied to (portions of) the web in a "striated" pattern, rather than exactly evenly and uniformly as is desired. As to "solvent-incompatibility", the need for a resilient (rubber, or other elastomeric) drive roll (D.sub.b) means only that aqueous base mixtures may be so applied, since the organic vehicle in a solvent-base mixture will attack an elastomeric roll surface and soon degrade it (e.g., dissolving "pit" sites on its surface, leading to uneven contact with the web, etc.).
The present invention is adapted to improve upon such delivery system and to ameliorate, if not eliminate, associated coating problems, especially for ultra-thin solvent-base coatings.